Hydraulic motors of the piston-type, e.g. axial-piston motors, may have one or more pistons received in a cylinder drum in respective cylinders which are hydraulically pressurized to press the pistons against an inclined surface. The surface may rotate with a shaft and thus can be termed a drive flange or can be fixed, the shaft being connected to the drum so that the reaction forces rotate the latter.
In both such motors, frictional retardation between the piston and the wall of the cylinder develop during operation (sliding friction) and exist before the rotation commences (static friction).
In motors having an inclined surface it has been common practice to provide a fluid cushion between a shoe at the head of the piston and connected thereto by a ball joint, the cushion being supplied with hydraulic fluid which is also passed into the ball joint. The fluid acts as a friction-reducing medium by forming a pressure film and as a lubricant.
During rotation of the drum, the engagement of the shoe with the inclined surface is practically frictionless because of the pressurized cushion although, before rotation begins and even after the cylinders of the drum have been pressurized, there is a static friction because the lubricating film has not been fully formed.
Because the static friction is substantially higher than the sliding friction encountered during rotation of the shaft and normal operations of the motor, initially a substantially higher torque must be developed to "break loose" the motor and begin its rotation. If the motor is dimensioned to deliver its maximum operating torque at a particular fluid pressure, this pressure is frequently not sufficient to break loose the latter moving parts of the motor against the retardation of the static friction, especially since static friction is often proportional to applied pressure. Consequently, the motor must be over-"dimensioned" i.e. dimensioned to deliver a higher torque than is normally required during its operation, for starting purposes, or simply may not be self-starting with the application of pressure.
The problem is especially pronounced in sliding-shoe motors in which the inclined surface is angularly fixed, because the initial pressurization of the cylinders applies the shoe with considerable force against the surface engaged thereby with static friction.
While the pressure cushion between the shoe and the inclined surface does tend to reduce the static friction, the problem nevertheless remains and friction between the piston and the cylinder wall is not reduced by such pressurization.
It has been proposed to couple the motor with the load by a clutch so that the starting torque can be applied without loading. This system has the disadvantage that additional elements are required in the drive train.
None of the solutions to the problem of starting a hydraulic motor against the static friction forces heretofore have proved to be fully satisfactory.